My Sister-in-Law Who Suddenly Appeared Was Not the Dreamy Airhead I Expected

A story about an illegitimate child taken in by a noble family who turned out to be eccentric. It's a heartwarming story about step-sisters that doesn't become antagonistic. [Added on 20250819] Thank you so much for the rankings! I was away from home for a few days during Obon and only found out about it afterward, so I was genuinely shocked... Thank you so much for reading! Also, thank you so much for reporting the typos... It's a huge help! I'm sorry, but I'm truly grateful to be reading your comments right now...! Thank you so much!